We have developed a new technique to better understand what happens to the microstructure inside a tablet during rapid disintegration.

Limitations of Disintegration Testing

In traditional disintegration testing it is difficult to establish any detailed insights into the mechanism of tablet disintegration as the test is merely designed to indicate the time it takes for a tablet or capsule to disintegrate completely, and this is defined as the state "in which any residue of the unit [...] is a soft mass having no palpably firm core". Based on the results of the disintegration test it is judged whether or not the dosage form meets the specification required by the respective pharmacopoeia e.g. for immediate release formulations. Apart from establishing the conformity with such official guidelines the disintegration test yields little additional information and is not very useful to guide rational formulation design.

{xtypo_quote_right}New insights for immediate release formulations and new opportunities for PAT measurement techniques{/xtypo_quote_right}

Fast and Non-destructive Imaging of Disintegration

Yassin et al. have introduced an alternative method based on terahertz pulsed imaging (TPI) to advance the understanding of how excipients, the dosage form microstructure and the testing conditions affect the disintegration behaviour in immediate release formulations [1]. The method allows for the first time to quantify the disintegration process on time-scales of seconds by measuring the ingress of the dissolution medium into a tablet with high precision and accuracy. Using this data the authors demonstrate that the disintegration process can be explained using theoretical models much like what is known for controlled release dosage forms. It is possible to investigate in detail how subtle changes in disintegrant concentration or the temperature of the dissolution medium affect the disintegration behaviour.

Figure 1: Disintegration process measured using TPI. The method can resolve both the swelling of the tablet as well as the ingress of the dissolution medium into the tablet.

Subtle Differences in Formulation have Profound Effects

A change in crosscarmellose sodium concentration from 2 to 5 wt% has a dramatic effect on the disintegration kinetics, particularly at a water temperature of 20°C. Here the disintegration time of the tablet is one order of magnitude faster at the higher concentration of superdisintegrant.

The study also highlights the enormous effect of the temperature of the dissolution medium on how rapidly a tablet disintegrates: by changing the temperature from 37°C to 20°C the disintegration time reduced from 25 to 5 seconds in a tablet containing 5% croscarmellose sodium (see Figure 2 below).

Figure 2: Disintegration characteristics of tablets made from MCC and superdisintegrant at different concentrations and water temperature (modified from [1]).

The new method is universally applicable to a wide range of formulations for dosage forms with disintegration times from seconds to hours.

PAT Measurements of Porosity – Non-destructive and Meaningful

In addition the paper highlights that measuring the tablet porosity instead of its hardness is potentially a much better PAT method compared to the time consuming and destructive weight/thickness/hardness testing.

It was previously demonstrated that terahertz spectroscopy is an excellent and very promising tool to non-destructively determine the bulk porosity of a tablet in a simple transmission or reflection experiment [2]. Yassin et al. show that such porosity measurements are very sensitive in resolving the disintegration performance of an actual tablet. Given that the terahertz measurements can be performed on millisecond timescales this technology could be developed into a powerful at-line/on-line or even in-line PAT technique.

Figure 3 (left): THz refractive index can be used as a PAT tool to measure the tablet bulk porosity (modified from [2]).

Non-linear Optical Imaging

Non-linear optical imaging is an emerging technique for imaging drugs and dosage forms [1]. Non-linear optical imaging may be used for non-destructive, non-contact imaging of solid drugs and dosage forms. It offers chemical and structural specificity with no requirement for labels, sub-micron spatial resolution (inherent confocal nature), rapid video-rate image acquisition, and the ability to image samples in aqueous environments in situ.

These combined features make non-linear optical imaging unique compared to existing imaging approaches in the pharmaceutical setting and make the technique well suited to a wide range of solid-state formulation and drug delivery analyses. These include imaging chemical and solid-state form distributions in dosage forms, drug release and dosage form digestion, and drug and micro/nanoparticle distribution in tissues and within live cells. While non-linear optical imaging is comparatively well established in the biomedical field, pharmaceutical applications of non-linear optical imaging are much less widely explored.

Principle of Non-linear Optical Imaging

Non-linear optical imaging involves irradiation of a sample with laser light (at one or two wavelengths) through an optical microscope and detection of scattered light at a different frequency. Non-linear optical imaging is also sometimes referred to as multi-photon imaging since the non-linear processes involve several photons (Figure 1). The technique encompasses a range of non-linear optical phenomena including second harmonic generation (SHG), coherent anti-Stokes Raman scattering (CARS) and two-photon fluorescence (TPF). In SHG, the energy of two photons is combined to emit light at half the laser wavelength. This process depends on the structural symmetry of the sample, and can be used to resolve crystalline and amorphous materials and some different polymorphic forms. In CARS, three photons at two or three wavelengths interact to efficiently generate light at a shorter wavelength (anti-Stokes Raman scattering). The technique is related to normal (spontaneous) Raman imaging, and is also used for label-free chemically-selective imaging. However CARS imaging is orders of magnitude faster, the spatial resolution is usually better and interference from fluorescence may be avoided. TPF is related to normal (one-photon) fluorescence, but it involves the energy of two incident photons instead of one with the advantage of being inherently confocal. Some materials (e.g. indomethacin, doxorubicin) generate TPF and so can be imaged with this technique without the requirement for labels. Vibrational energy level diagrams representing the SHG, CARS and TPF processes are shown in Figure 1.

Since the non-linear optical phenomena have different advantages and specificities, it is often very helpful to collect a combination of these signals at the same time with the same imaging setup. This is known as "multi-modal" imaging.

Imaging Solid Drugs and Dosage Forms

It is becoming widely recognised that critical solid dosage form properties, such as drug dissolution and release, are dependent not only on the formulation composition but also the component and solid state form distribution. Non-linear optical imaging is well suited to imaging a range of dosage forms. It is capable of rapidly imaging different chemical components and solid forms with high resolution (micron or sub-micron) in three dimensions. In general the data for the images may be collected in a few seconds or less. The technique may also be used to image changes in dosage forms in situ during drug release/dissolution and storage [2].

Distributions of components in tablets may be imaged in 2D or 3D, as shown in Figures 2 and 3. Both drug and excipient distributions may be imaged.

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Future Work

As mentioned above, the technique is suited for real time imaging of drug release/dissolution. In collaboration with the Optical Sciences Group, University of Twente, The Netherlands and Institute of Pharmaceutics and Biopharmaceutics, Heinrich Heine University, Duesseldorf, Germany we are currently working on imaging drug and dosage form changes in a flow-through cell while simultaneously analysing drug concentration in solution.

Non-linear optical imaging is also well suited to real-time imaging of cells and tissues. We are currently working on imaging delivery of poorly water soluble drugs in various types of formulations in vitro and in vivo. If feasible, this approach will facilitate bringing together the analysis of drug release/dissolution and permeability, and should help lead to better understanding of absorption of these drugs.

PSSRC Facilities

Asst. Prof. Clare Strachan (Division of Pharmaceutical Technology, Faculty of Pharmacy, University of Helsinki) has several years' experience in non-linear optical imaging of a range of solid dosage forms. A fully integrated commercial non-linear optical microscope (Leica TCS SP8 CARS microscope) is available at the University of Helsinki. This is the first commercially available fully integrated CARS microscope in the world. The microscope uses a picosecond solid-state-laser light source to excite single Raman lines within a range of 1250 cm-1 to 3200 cm-1 for CARS imaging. It gives access to molecular specific contrast based on a variety of Raman-active vibrations relevant to pharmaceutical applications. Second harmonic generation (SHG) and two-photon fluorescence (TPF) are also possible with the setup. The microscope is also capable of one-photon fluorescent imaging in the UV and visible wavelengths. All non-linear and fluorescence phenomena can be imaged on exactly the same sample with the same microscope, and therefore a direct comparison of the imaging approaches can be made.

Introduction

Oral films have gained interest in the last couple of years. Films for oral application offer an interesting new approach for drug administration. Active pharmaceutical ingredients (API) can be implemented in thin-sheeted polymer film matrices. These dosage forms are intended to be placed in mouth to dissolve in the saliva without the need of additional liquid and without swallowing of a solid dosage form.

Definition - Oromucosal Film Preparations

Most recently, films for oral drug delivery became part of the European Pharmacopoeia, edition 7.4 [1]. They are subordinated to the monograph 'oromucosal preparations'. Films are either described as fast-dissolving 'orodispersible films' or 'mucoadhesive / buccal films', which are intended to be attached to oromucosal sites.

Application

Oral film preparations are useful dosage forms for local drug administration, but also for systemic drug delivery (Fig. 1). A bioadhesive film can be placed on the oromucosal tissue. By film dissolving the API is released onto, into or through the mucosa. Assuming absorption of the API, action may take place by avoiding the gastrointestinal and the enterohepatic route. The use of multiple layers in a mucoadhesive film preparation is reasonable, when pursuing the setup of a multifunctional system (Fig. 2). However, films can also function as alternative per oral administration. A fast-dissolving orodispersible film that is placed on the tongue disintegrates within seconds. Subsequently, the released API is intentionally or incidentally swallowed with the saliva [2].

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Manufacturing of Films

The most popular approach to manufacture films is the solvent casting method. Viscous solutions made of polymers (e.g. cellulose derivatives, polysaccharides) and dissolved or suspended API are cast on a film application apparatus by the help of a coating knife [3]. After drying and solvent evaporation, films can be cut into single doses [4]. Wet and melt extrusion offer an alternative way of manufacturing. Polymer mass is pressed through a die, which has a laminar opening at best, to obtain film stripes. An alternative and flexible manufacturing method of drug-loaded film preparations are drug printing methodologies, such as Flexography [5] and ink-jet printing [6].

Challenges in Characterisation

Novelty of the pharmacopoeial monograph leads to a lack in characterization requirements. According to the Ph. Eur. only adequate drug release and mechanical strength are required to be tested without providing detailed instructions. Previous research projects already focused on new characterization methods for films [5]. Other than for orodispersible tablets, there is no time restriction for film disintegration so far. Even if the monograph describes mucoadhesive dosage forms, there is no standardized method to proof bioadhesion appropriately. As oromucosal film preparations are intended to dissolve in mouth, taste of the formulation presents another challenge in dosage form development. Therefore, masking the taste of a bitter drug can be achieved by applying different taste-masking strategies. Taste-masking effects may be assessed in-vitro by the use of electronic taste sensing systems [2, 7].

{xtypo_quote_right}Oromucosal films are an attractive type of formulation for personalised medicine.{/xtypo_quote_right}

Future Work

The research on oromucosal film preparations in Düsseldorf is further focusing on the development of new manufacturing and improved characterization methods for film dosage forms. The professional expertise in film manufacturing was recently expanded to optimize the attributes of film preparations with suspended APIs, gastro intestinally unstable APIs and multiple layered oromucosal films.

PSSRC Facilities

The research groups of Professor Jörg Breitkreutz and Professor Peter Kleinebudde in Düsseldorf are working on solid dosage forms and pharmaceutical processes like roll compaction / dry granulation, extrusion and coating. Drug development for neglected patients such as pediatric and geriatric subpopulations are of key interest. A major topic is the development and characterization of new orodispersible film preparations, which is performed in the focus groups 'Coating and Films' chaired by Dr. Klaus Knop and 'Advanced Analytics' chaired by Dr. Miriam Pein.

Magnetic Resonance Imaging

The use of MRI as a powerful imaging and characterization modality in pharmaceutical dissolution research is now well established [1]. The non-invasive and non-destructive nature of MRI enables the investigation of structural, chemical and dynamical processes in many optically opaque systems at the microscopic level. Spatial maps of water penetration, tablet swelling and dissolution, as well as the mobilization and distribution of drug products can now be quantified and visualized [2,3]. In addition, the hydrodynamics within a USP recommended flow-through dissolution apparatus can also be visualized by MRI [4]. Such comprehensive information is essential in pharmaceutical research for: (i) the correct interpretation of conventional drug dissolution profiles and (ii) the optimal design (QbD) of controlled release formulations.

MRI Principle

Magnetic resonance images of a sample are reconstructed from a nuclear magnetic resonance (NMR) signal, which is generated by certain nuclei (most commonly 1H) when subjected to a strong external magnetic field, B0, (e.g. 9.4 Tesla) and subsequently irradiated with radio frequency pulses. A spatially encoded NMR signal, i.e. an image, is first generated by the application of RF pulses and additional much smaller magnitude magnetic field gradients. The spatial image can then be obtained via Fourier transformation of the raw data. Figure 1 depicts the set-up of a vertical MRI magnet and USP-IV dissolution cell.

By tailoring the timings of the radio frequency pulses and magnetic field gradients, the MR images can be weighted to show different information such as the chemical composition, spin-lattice relaxation time (T1), spin-spin relaxation time (T2), and molecular self-diffusion coefficient, as well as the velocity of flowing dissolution media within the dissolution apparatus.

The ‘quantitative’ nature of magnetic resonance is one of the defining beauties of MRI. The acquired signal, in theory, is proportional to the number of nuclei of interest in a particular sample. Thus MRI tells us ‘how much’ of a particular substance we have in a particular system [1-3,5]. For example, we can spatially map the concentration of water, or the API in a swollen gel layer (see Figure 2).

In addition to ‘how much’, MRI data can be also be acquired and manipulated to give quantitative information regarding ‘how fast’ the molecules of interest move [1-3,5]. For example, of particular interest within the pharmaceutical research community is being able to: (i) quantify the rate of ingress of dissolution media into swellable matrices and (ii) quantify the rate of formation and expansion of gel layers (see Figure 2).

{xtypo_quote_right}Quantitative MRI provides unique insight into the change in tablet microstructure during dissolution.{/xtypo_quote_right}

Hence by using the comprehensive MRI based information of the tablet swelling process during dissolution, it is inturn possible to evaluate the polymer structure quantitatively. For example, we have found two distinct regimes in the ‘gel’ region, namely the ‘swollen glassy layer’ (SGL) and the ‘gel layer’, based on the correlation between the water concentration and T2 obtained by MRI (see Figures 3 and 4). The temporal evolution of each component can thus be monitored accordingly (see Figure 5).

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The movie shows how the in-vitro T2-relaxation time maps can be used to follow the evolution of the gel and swollen glassy layers with time and highlights the quite different behaviour of the formulations chosen here.

Interpreting drug release profiles

The determination of the gel structure and the evolution of each component are essential pieces of information because the water ingress and polymer swelling directly affect the drug release process. In particular, the definition of swollen glassy layer and its separation from the gel layer are critical to aid our understanding of drug release, because the HPMC polymer chains start to relax in the SGL and create larger voids for the drug to diffuse through.

A case study is illustrated in Figure 6. Three grades of HPMC with different molecular weights (K100M > K15M > K4M) were compared as swelling excipients for the release of model drug ibuprofen (IBU). It is found that the release rate of IBU from K4M is the highest, while K15M and K100M have similar release profiles. Without the differentiation of the SGL from gel layer, the comparison of total gel region (SGL+ gel layer) shows that K4M has the largest swollen layer, which in theory results in the slowest release rate. However this is contrary to the observed cumulative drug release profile. With the evaluation of a separate SGL and gel layer, it is clearly seen that the SGL of K4M disappears fastest, indicating the drug becomes fully hydrated in the shortest time. Despite the fact that the IBU from the K4M sample has the thickest gel layer to diffuse through, it still releases the fastest. Thus, in this case, the rate determining step for release of IBU from HPMC matrix is controlled by the rate of HPMC hydration.

Future work

The majority of current MRI research studies in the pharmaceutical community acquire signals from water molecules and very few studies have investigated directly the behaviour of the APIs, since the 1H signal from API is normally obscured by the huge 1H signal associated with the water based dissolution medium [2]. We are currently exploring the 2D imaging of the API using the signatory atoms it possesses. Preliminary results indicate that MRI shows great potential in revealing the distribution and evolution of APIs under in vitro pharmacopeial dissolution conditions. The combination of NMR and MRI techniques applied to API and dissolution media can bring together the imaging results of both species, which will certainly result in a more comprehensive understanding of the controlled release systems.

PSSRC Facilities

The group of Professor Lynn Gladden and Dr Mick Mantle at the Magnetic Resonance Research Centre (MRRC), University of Cambridge. The MRRC acts as a focus for applications of magnetic resonance techniques in chemical engineering research in the UK. The main research interest is in understanding multi-component adsorption, diffusion and flow processes. These phenomena are particularly important in the controlled release of pharmaceuticals, one of the focus areas of the research at the MRRC. The centre has wide expertise in the study of diffusion processes into pharmaceutical matrices and coatings by magnetic resonance imaging (MRI). A number of research projects at the MRRC are in collaboration with major pharmaceutical companies.

Lipid-based drug delivery systems have become a popular approach for the delivery of poorly water-soluble drugs. The limitations associated with this formulation strategy have been the drug solubility in the delivery systems and the lack of characterization techniques predicting the in vivo performance. Solid state characterization of the in vitro digestion products has provided new insights that scrutinize current paradigms in the development of lipid-based drug delivery systems.

Lipid-based drug delivery

Lipid-based drug delivery systems take advantage of poorly water-soluble drugs being presented in the dissolved state, thereby avoiding the often limiting dissolution rate of solid, crystalline drugs [1]. The current development of lipid-based delivery systems, in particular self-nanoemulsifying drug delivery systems (SNEDDS), involves the assessment of drug solubility in excipients and mixtures [2]. The construction of a phase diagram and dispersion of water-free SNEDDS pre-concentrates help identify mixtures capable of generating nano-emulsions upon contact with aqueous medium under gentle stirring [3]. Thereafter, the identified SNEDDS are subjected to dynamic in vitro lipolysis. In essence, the in vitro digestion model simulates the physiological degradation of lipids by pancreatic lipase and co-lipase in the gastrointestinal tract (Fig. 1a). Samples collected during in vitro lipolysis are quantified for drug in the aqueous phase and in the pellet obtained after a centrifugation step (Fig. 1b).

Until recently the common notion was that only the drug solubilized in the aqueous phase was available for absorption. Conversely, the presence of precipitated drug in the pellet has been considered as undesired in the development of SNEDDS. The rationale behind this notion was that the limited dissolution rate of solid compound would be re-introduced with precipitation, compensating the initial advantage of the dissolved drug present in the pre-concentrate [5-7]. However, this implies that the drug precipitates in a crystalline form. That this is not necessarily the case has been shown by Sassene et al. who investigated the isolated pellet obtained after in vitro lipolysis by X-ray powder diffraction (XRPD) and polarizing light microscopy (PLM) [8]. In fact, the authors observed that cinnarizine precipitated in an amorphous form. Moreover, the dissolution rate of the precipitated cinnarizine was substantially increased compared with the crystalline starting material. So far the possible implications of this finding for the absorption in vivo were not known.

{gallery}lipids{/gallery}

Supersaturated SNEDDS

Pre-concentrates loaded with amounts of simvastatin exceeding the drugs' solubility in the pre-concentrates (i.e. supersaturated SNEDDS, or super-SNEDDS, Fig. 2) were developed and subjected to dynamic in vitro lipolysis according to the accepted digestion protocol established in Copenhagen [9, 10].

Precipitation of simvastatin during in vitro lipolysis was evident and, as seen for cinnarizine, XRPD analyses revealed that the precipitate contained amorphous simvastatin (Fig. 3). Following administration of the super-SNEDDS to beagle dogs the bioavailability of simvastatin increased significantly to 1.8-fold of the bioavailability of dose-equivalent conventional SNEDDS [11]. Importantly, improved bioavailability was observed despite drug precipitation in vitro. It was speculated that the rapid dissolution rate of amorphous simvastatin was able to contribute to the overall bioavailability of the drug. This hypothesis was supported by an increased half-life of elimination calculated for super-SNEDDS compared to conventional SNEDDS (2.3 hours and 1.4 hours, respectively).

In another study an amorphous precipitate was also found for super-SNEDDS containing the antimalarial drug halofantrine [12]. The precipitation during in vitro lipolysis was not reflected in a reduced bioavailability. These results emphasize the need to investigate the solid state properties of precipitates generated during in vitro lipolysis.

Implications of the study

The studies have shown the feasibility of super-SNEDDS containing increased drug loads without compromising bioavailability. Moreover, previous concerns about drug precipitation need to be revised. The use of solid-state techniques has proven complementary to in vitro lipolysis and should be used routinely for the characterization of lipid-based drug delivery systems.

PSSRC Facilities

The group of Prof Thomas Rades in Copenhagen has extensive experience in solid state characterization of drugs. The group's knowledge has recently been complemented by the expertise of A/Prof Anette Müllertz in the field of biopharmaceutics of lipid-based drug delivery systems. The combination of physical chemistry with the formulation of novel dosage forms underpins the group's translational approach to research. Micro-dissolution testing, in vitro lipolysis, and access to animal facilities complete solid state equipment such as high-throughput stage XRPD.